Traditional four-stroke cycle engines are configured with one or more cylinders wherein each one of the cylinders goes through all the four strokes (intake, compression, combustion and exhaust) of a thermodynamic cycle. This basic century old arrangement is still used in a modern vehicle because of its simple construction and efficiency to produce power that causes a vehicle move. But in present day's scenario wherein the ever depleting petroleum resources and alarmingly increasing CO2 in global atmosphere insists the scientists to rethink on the traditional energy conversion technologies, the Internal combustion (IC) engines need to be more fuel efficient and less environment hazardous. In spark ignition (SI) engine, there are various practical constraints in the traditional engine design that produces poor overall thermodynamic efficiency, especially at regular drive conditions of a vehicle. Because the SI engine load control is essentially done by quantitative control in induction of combustible mixture, the regular drive condition or low engine load condition in a SI engine suffers from various problems like: 1) considerable charge dilution and increase in induction fluid temperature by residual burnt gas wherein, higher induction temperature limits compression ability of the working fluid, 2) low initial and peak combustion chamber pressure, 3) slow flame propagation in combustion chamber, 4) incomplete combustion and 5) pumping loss.
The basic components of an internal combustion engine are well known in the art and include the engine block, cylinder head, cylinders, pistons, valves, camshaft and crankshaft. The cylinders, cylinder heads and tops of the pistons typically form working chambers into which fuel and air is introduced and combustion takes place. The volumes of the working chambers or chamber volumes repetitively expand and contract with the back-and-forth motion of the pistons. In a four-stroke cycle engine, power is recovered from the combustion process in four separate piston strokes of a single piston. The piston is so connected to a crankshaft by a connecting rod that the back-and-forth motion of the piston can be translated into rotary motion of the crankshaft. A stroke is defined as a complete movement of a piston from a top dead center (TDC) position to a bottom dead center (BDC) position or vice versa. The strokes are referred to as intake stroke, compression stroke, combustion or expansion stroke and exhaust stroke. Wherein, only the expansion stroke is the power delivering stroke that causes a vehicle move. All the remaining strokes are power consuming strokes. When the piston reaches to the top dead center (TDC) position the chamber volume contracts to its minimum value and at the bottom dead center (BDC) position of the piston the chamber volume expands to its maximum value. The minimum chamber volume also referred to as the clearance volume. Ratio of the maximum and minimum chamber volumes represents the engine's compression ratio which is fixed for a conventional engine. The efficiency of an SI engine substantially relies on its compression ratio that means higher the compression ratio, higher the engine's thermodynamic efficiency. Higher compression ratio produces higher combustion chamber pressure and temperature and thereby results in more heat conversion to useful work. Though, beyond certain restriction point the compression ratio induces knocking which is detrimental to the engine. Knocking means a high pressure wave generated by uncontrolled combustion in SI engine's combustion chamber and this phenomenon greatly rely on the initial combustion chamber temperature, pressure and compression ratio of the working volume. Therefore, the compression ratio of an SI engine is determined by considering this knocking point.
The load control of a spark ignition (SI) engine is done by controlling the induction of fuel-air mixture quantitatively. Therefore, at common drive condition, SI engine cylinders are charged with only a fraction of air-fuel mixture than that of its optimum capacity. The quantitative control of fuel-air mixture is done by throttling the intake passage, therefore the pressure in the intake passage drops significantly below the atmospheric pressure and the piston has to do some additional work in intake stroke which is generally known as pumping loss. As a result, the initial and final combustion chamber pressure drops drastically and this phenomenon affects the cycle thermodynamic efficiency. At the end of every thermodynamic cycle, some nearly constant amount of burnt gas residues remain in the clearance volume of the cylinders and in the next cycle this inert residual gas mixes with fresh intake gases and makes it diluted. At ordinary drive condition this residual gas proportion is substantially higher than it is at heavy load drive condition; hence the charge become considerably diluted and this reduces the flame speed in working fluid and results in poor combustion quality. Dilution also increases the chances to misfire and so fuel enrichment is needed.
Traditional SI engines intake and compress a mixture of fuel and air. The ratio of specific heat (γ) of fuel-air mixture is considerably less than that of only air. It is evident to those familiar in the internal combustion engine thermodynamics that the working fluid with higher ratio of specific heat produces higher cycle efficiency. This is one of the reasons behind the greater efficiency of Compression Ignition (CI) engine over Spark Ignition (SI) engine. Some modern engine manufacturers using Gasoline Direct Injection (GDI) technology wherein, at low-load drive conditions GDI technology uses only air as intake fluid and fuel is injected at the later stage of compression phase. GDI technology also uses stratified charging method that forms fuel rich mixture at sparkplug vicinity and fuel lean mixture at rest of the area, wherein maintaining the overall mixture fuel lean. The ratio of specific heats of fuel lean mixture is higher than stoichiometric (chemically correct) mixture, hence, produce greater thermodynamic efficiency. Moreover, at regular drive conditions GDI can reduce the need of throttling and thereby the pumping loss also. But, fuel lean combustion deteriorates the performance of Three-way Catalytic Converter (TWC). GDI also needs costly fuel injectors and precise control system.
It is known that a spark ignition (SI) internal combustion (IC) engine is generally most efficient when the cylinder pressure and temperature at the end of a compression phase are closed to its maximum tolerable limit. In a conventional spark ignition engine, this condition is achievable only when the throttle valve in the intake manifold is fully open to allow the maximum possible air or fuel-air mixture in the engine cylinder during intake phase and during following compression phase said intake air get compressed into a minimum chamber volume which is fixed by the design of the engine. During fully-open throttle condition the intake manifold pressure is near atmospheric pressure or about 1 bar. During the typical driving conditions which generally cover above 90% of the entire drive cycle, the intake manifold pressure remains about 0.5 bar or less, causing considerable drag on the driveshaft and this phenomenon is commonly known as ‘pumping loss’, that adversely affects the engine efficiency. Throttling further reduces chamber pressure and temperature at the end of compression phase and increase charge dilution. Hence reduces the combustion flame speed and the engine suffers from unstable combustion which leads to reduction in efficiency and increase in hazardous tailpipe emissions.
Conventionally, a mid-size car with a gasoline engine is only about 20% efficient when cruising on a level road whereas the rated peak efficiency of the car is about 33%. That is, during cruising, the Specific Fuel Consumption (SFC) of the engine is about 400 g/kWh, while under high load condition the same engine can reach a SFC of 255 g/kWh. See, P. Leduc, B. Dubar, A. Ranini and G. Monnier, “Downsizing of Gasoline Engine: an Efficient Way to Reduce CO2 Emissions”, Oil & Gas Science and Technology—Rev. IFP, Vol. 58 (2003), No. 1, pp. 117-118. As the engine operating condition goes below cruising mode such as the city driving conditions, the efficiency further reduces drastically. Considering this, if an engine is so downsized to operate with higher specific load during cruising or city driving condition, it could not accelerate or climb steep road well.
In the past decades some interesting ideas like Variable Displacement Technology, Variable Compression Ratio Technology, Variable Valve Technology, Engine Downsizing and Pressure Boosting, Stratified Charging of Fuel, Controlled Auto Ignition, Load Dependant Octane Enhancement of Fuel have been introduced in order to attain better SI engine efficiency and various sets of combinations of these methods have also been experimented within a single engine.
In reciprocating piston Spark ignition engine, the Variable Displacement volume of engine is generally achieved by cylinder deactivation method, wherein, during part load operation, few cylinders of a multi-cylinder engine are selectively deactivated so that not to contribute to the power and thus reducing the active displacement of the engine. Therefore, only the active cylinders consume fuel and are operated under higher specific load than that of the all cylinder operations, hence the engine attains higher fuel efficiency. The number of deactivated cylinders can be chosen in order to match the engine load, which is often referred to as “displacement on demand”. As pistons of both of the active and deactivated cylinders are generally connected to a common crankshaft, the deactivated pistons continue to reciprocate within the respective cylinders resulting in undesired friction. The valves of the deactivated cylinders need specialized controls, which produce further complications. Moreover, the deactivation and reactivation of cylinders take place in steps, and therefore further measures become necessary in order to make the stepped transitions smooth. Managing unbalanced cooling and vibration of variable-displacement engines are other designing challenges for this method. In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. Modern electronic engine control systems are configured to electronically control various components such as throttle valves, spark timing, intake-exhaust valves etc. in order to smoothing of the transition steps of a variable displacement IC engine. An example of electronic throttle control method is to be found in U.S. Pat. No. 6,619,267 (Pao), describing the intake flow control scheme to manage the transition steps. A variable displacement system for both the reciprocating piston and rotary IC engines is disclosed in U.S. Pat. No. 6,640,543 (Seal) that includes a turbocharger to enhance the working efficiency.
Like variable displacement engine technologies, the variable compression ratio (VCR) technologies also require various associated modifications such as engine downsizing, turbocharging or supercharging, variable valve technology, load responsive octane enhancement of fuel etc. to meet increasing stringent emission norms and fuel efficiency requirements. The basic VCR idea is to run an engine at higher compression ratio under part load operating conditions when a fraction of full intake capacity is consumed and at relatively lower compression ratio under heavy load conditions when the full intake capacity is consumed. Thereby the resultant cylinder pressure and temperature at the end of compression can be improved through a wide load conditions, hence, better fuel efficiency could be achieved. As VCR technology alone cannot avoid part load pumping losses, it requires assistance of Variable Valve Technology (VVT). The VVT provides the benefit of un-throttled intake to an SI engine, wherein the amount of intake gas at part load is controlled by either closing the intake valve early to stop excess intake or by late intake valve closing so that to discharge excess intake gas back to the intake manifold. The VCR technology itself, however, is quite complex to design and manufacture. See “Benefits and Challenges of Variable Compression Ratio (VCR)”, Martyn Roberts, SAE Technical Paper No. 2003-01-0398.
Over expansion cycle in a SI engine can add significant benefit to its thermal efficiency. The Atkinson cycle and Miller cycle efficiency is established on the said over expansion cycle principle, see “Effect of over-expansion cycle in a spark-ignition engine using late-closing of intake valve and its thermodynamic consideration of the mechanism”, S. Shiga, Y. Hirooka, Y. Miyashita, S. Yagi, H. T. C. Machacon, T. Karasawa and H. Nakamura, International Journal of Automotive Technology, Vol. 2, No. 1, pp. 1-7 (2001). The over-expansion cycle can produce substantial benefit in thermal efficiency over conventional engine cycle when being applied together with variable compression ratio and variable valve technology. But the introduction difficulties remain too high to introduce in a practicable engine.
Various specialized prior art engines have been designed in an attempt to increase engine efficiency. By way of example, a recent prior art engine is described in U.S. Pat. No. 7,628,126 to Carmelo J. Scuderi entitled “Split four stroke engine”. In this engine, a crankshaft rotating about a crankshaft axis of the engine. A power piston is slidably received within a first cylinder and operatively connected to the crankshaft such that the power piston reciprocates through a power stroke and an exhaust stroke of a four stroke cycle during a single rotation of the crankshaft. A compression piston is slidably received within a second cylinder and operatively connected to the crankshaft such, that the compression piston reciprocates through an intake stroke and a compression stroke of the same four stroke cycle during the same rotation of the crankshaft. A gas passage interconnects the first and second cylinders. The gas passage includes an inlet valve and an outlet valve defining a pressure chamber therebetween. The outlet valve permits substantially one-way flow of compressed gas from the pressure chamber to the first cylinder. Combustion is initiated in the first cylinder between 0 degrees and 40 degrees of rotation of the crankshaft after the power piston has reached its top dead center position.
In this engine, at the end of a compression stroke, the combustion initiates in the first cylinder and being connected with the same crankshaft, the phase relation of the power and compression piston is fixed. Therefore, at the point of ignition the combustion chamber volume is fixed for all load conditions and this should essentially be optimized for the full load driving condition. At typical drive conditions, when the engine consumes a fraction of its full intake capacity, the initial pressure and temperature of the expansion chamber should drop drastically. This phenomenon should affect the engine's part-load performance.
Another prior art engine is described in U.S. Pat. No. 7,353,786 to Salvatore C. Scuderi entitled “Split-cycle air hybrid engine”. Various operating modes and alternative embodiments of the engine are described, in which at part load operating mode of the engine a fraction of total compressed air is used for combustion purpose and the rest is stored in a storage tank for future uses. The volume compression ratio of both the compression and power cylinders of this engine is very high (80 to 1 or more). Therefore, at part load mode when only a fraction of compressed gas is used for combustion, the combustion chamber shape at the time of ignition would be very thin if a favorable chamber pressure and temperature is maintained and this kind of chamber shape is highly unfavorable to carryout a desirable combustion. Moreover, it is very difficult to retain the temperature and pressure of compressed air stored in the storage tank and so using of the stored compressed air would be very difficult due to its continuously variable pressure-temperature parameters.
Accordingly, there is a need for an improved four-stroke spark ignition internal combustion engine, which is simple to manufacture and can maintain favorable combustion chamber conditions, e.g. suitable combustion chamber pressure, temperature, turbulence and chamber shape at all the driving conditions. The engine should be an over expansion cycle engine and capable to carryout such a charging method that enhance engine's thermodynamic efficiency.